I wonder why some sims don't change their clothes.

I have no idea - every sim I play changes their clothes if you have their outfits all picked out. I generally pre-plan 5 outfits in each category - and a week or so later when I tire of those I pre-plan another 5 outfits in each category. But my sims constantly change - to fit where they are going usually without me even telling them to.

I don't use mods or CC - do you? Sometimes they can create issues in game mechanics like that if there are issues with CC or mods. But I am often amazed at how often my sims change.

The only thing that irks me is sometimes when out at a festival and wearing what I am glad they have on - they suddenly see the basketball hoop and decide to change into sports wear and play ball when they are there at the festival to hopefully find a new love. That is very annoying. Especially if the sim is a heir in one of your legacy's and finding a mate is something the sim has to do - that I can't do for him or her. (I play by Pinstars rules)

Yep that is really weird being in regular clothes in the pool or getting a massage. That does seem like a mod issue....